I would go for a nice jacket. I know it may not fit when you are 22-23, but that's another 4-5 years far into the future. At daily basis I do math, so as always I can do some math here as well:

You buy a jacket for £160 and it fits for let's say 4 years. That makes it £40 for each year to own a jacket.

Consider having a jacket for £40 a year, is'nt that worth it? Considering that now that you finally belongs to the rank of grown people, you will enjoy the last years of the youth with a kilt&jacket that gives you all the glory a kilt can give. (And you could even put it on for sale for £80 when you'r done with it; that makes it £20 a year)

Don't put me responsible, but this is the way I think when questions like these gets attached on my brain.